This invention relates generally to machine tools, and more particularly to a device and a method of guiding a machining electrode in a machine tool.
A wide variety of machine tools for machining work pieces are known. One example that can be mentioned here is a spark erosion machine, such as a wire erosion machine, which generally has the following design elements: (a) a machine frame with drive devices and a guidance system for a wire electrode, (b) a rinsing system, (c) a generator and (d) a numeric control. In wire erosion, a first electrode (the eroder wire) is used to cut a contour into or out of a second electrode (the work piece), starting from a starting bore or a reference surface. In doing so, the wire electrode is guided in an upper wire guidance head (i.e., arranged above the work piece) and a lower wire guidance head (i.e., arranged beneath the work piece). The wire running system is usually designed so that the wire electrode is continuously drawn off a supply roll, conveyed over a conveyance distance to the working zone and conveyed through the working zone between the guide heads and then disposed of. The wire running system and the machine control ensure a suitable wire draw-off speed and a suitable mechanical wire tension.
One problem of a special type exists with such machine tools. Specifically, due to the fact that before the actual machining, the machining electrode must be threaded with great precision in the working zone of the machine (i.e., the end of a wire-shaped machining electrode (wire electrode), for example, must be guided from the upper wire guidance head through a starting bore or a working gap in a work piece to be machined toward the lower wire guidance head). However, it is often necessary to thread a new wire electrode into the system during machining. This is the case, for example, when the overall machining is composed of multiple closed contours, in a case when closed contours are cut in several partial sections, or when different wire electrodes are used. In addition, it is possible for a wire to break during operation (e.g., in the case when there are problems in rinsing or when the materials of the wire electrode or the work piece are not homogeneous), which can lead to a concentration of discharges and, thus, to local mechanical and thermal overloading of the wire electrode.
To increase operating efficiency, automatic wire systems are preferred in modern wire erosion machines. Manual threading of the wire electrode can be performed only with great effort on wire erosion machines that operate with fine or extremely fine wire electrodes, because the wire is hardly visible and has practically no inherent stiffness. However, the automatic threading systems known in the state of the art do not completely meet the requirements of modern machine tools, especially in variable machining situations.
Mechanical methods were used in the past. In these methods, a wire electrode was conveyed by movable means from one wire guidance head to the other.
Swiss Patent 559,599 by the present applicant described, as early as 1973, a wire threading method in which the wire electrode can be guided with the help of a free concentric fluid jet (such as a high pressure jet of water), to overcome the distance from the upper wire guidance head to the lower wire guidance head. This constituted a considerable advance in automation of wire erosion machines. A similar method was described later in German Patent 3,126,444 C2. In this method, a suction effect is created at the outlet end of a starting bore in the work piece, thus forming a suction flow of the transport fluid at the entrance end of the bore, leading to intake of the wire. In the case of several starting bores in a work piece located close together, however, this yields the problem of threading the wire into a certain starting bore in a controlled manner.
To perform an optimal and reliable threading operation, different threading elements are necessary for different types of wire to produce a suitable threading jet. For example, the present applicant uses a number of adapted threading nozzles such as those described in German Utility Model 9,421,952 U1, to produce a fluid jet having different diameters of the jet for threading the wire electrode. These nozzles are inserted manually by the operator into the upper wire guide head. When working with different grades of wire, manual intervention is necessary in order to change nozzles.
Another automatic wire threading method is also known from European Patent 233,297 B1, where a separate threading device, which is connected to the upper wire guide head in an articulated manner, is used. For threading the wire, the upper wire guidance head is lifted and the wire threading device, which is equipped with a nozzle to produce a fluid jet, is pivoted into the wire guidance area. However, in the case of different grades of wire, especially with different wire diameters, it is often necessary to change the threading nozzle as well. In addition, another disadvantage is that a complicated mechanical wire threading device is necessary, which takes up a lot of space in the work room. A portion of the traversing distance of the Z axis is lost due to the upstream connection of the wire threading device. In addition, the pivoting movements of the wire threading device before and after threading the wire cause an increased risk of collision with the work piece or the chucking means in the working zone.
An additional challenge in guidance of a machining electrode through a fluid jet, especially for automatic threading, is the fact that only a satisfactory beam geometry guarantees reliable guidance of the electrode and thus a reliable threading operation. Even a minor disturbance can result in destruction of the fluid jet and thus to failure of the threading operation. For example, if the fluid jet comes in contact with the surface of the work piece, the inside wall of a starting bore or some other obstacle the threading operation may fail. The requirements of the automatic threading operation in the case of machine tools, especially in the case of wire erosion machines, have increased considerably over the decades. An example of this is the threading operations required with the contours of male and female molds, which have become increasingly more like filigree or open work and are to be produced by means of EDM or wire erosion. The finest wire electrodes in use today have a diameter of a few hundredths of a millimeter. The dimensions of the starting bore are also becoming smaller and smaller, so that they take up less space. Accordingly, the requirements of automatic threading have also increased as well. The diameter of a fluid jet used for this purpose should therefore be only slightly larger than the diameter of the wire electrode transported. Wire erosion machines, which are generally used universally, are usually operated with different grades of wire and wire diameters, depending on the application. Therefore, there is a need for adapting or optimizing the threading operation to different machining situations. Although a wire threading fluid jet may give satisfactory threading results with a thin wire, for example, the same fluid jet may under some circumstances be unsuitable for a thicker wire. In principle, this is true not only of threading operations but in general for conveyance operations in which a machining electrode is conveyed with the help of a concentric fluid yet.
The disclosed device and method for guiding a machining electrode are suitable for a broad spectrum of machining electrodes and/or machining situations. In particular, they provide a wire electrode threading device and a corresponding threading method that are suitable for various threading situations.
More specifically, the disclosed device for guiding a machining electrode such as a wire or strip electrode in a machine tool with the help of a fluid jet includes a feed-through channel for producing a fluid jet within which the machining electrode is guided, and one or more control devices for varying the cross section of the feed-through channel and, thus, the cross section of the fluid jet.
As mentioned above, only a satisfactory fluid jet geometry which is adapted to the respective machining electrode to be conveyed through the machine can guarantee reliable guidance of the machining electrode. The present inventors have recognized that the fluid jet diameter used in each case plays a crucial role. With the help of the device described herein, it is possible to adapt the cross section of the fluid jet to a broad spectrum of conveyance situations without otherwise having to intervene in the electrode transport system of the machine. Use of the disclosed guidance device for automatic threading of a machining electrode in a wire erosion machine is especially advantageous, because the device can be used universally for various threading situations with respect to the wire diameter, the diameter of the starting bore or the working gap width, the height of the work piece and/or other conditions involving the work piece.
The illustrated electrode guidance device is designed with a feed-through channel having a variable size. For threading a wire electrode in a wire erosion machine, the variable feed-through channel is in an area of the upper wire guidance head through which the wire electrode and a fluid under pressure are guided. The means for adjusting the feed-through channel and, thus, the threading jet cross section can be implemented in various ways, as will be explained in greater detail below. The control device or each control device preferably has at least one rigid control element which is movable in the feed-through channel to vary the cross section. This may be a linear or rotational control movement or a combination of the two. The control element is preferably inserted into the feed-through channel at the side, especially transversely. Alternatively, the control element is moved vertically (axially) or quasi-vertically relative to the feed-through channel.
The preferred embodiments have the advantage that the fluid jet, which directs the machining electrode in the desired direction as if between guide boards, can be optimally adapted to the diameter of the machining electrode used, the diameter of the starting bore in the work piece, the height of the work piece, etc. by varying the dimension of the feed-through channel. In the case of wire erosion in particular, wire electrodes having a wide variety of diameters are used in practice, depending on the type of erosion process and the material to be cut, especially in a diameter range of 0.03 mm to 0.35 mm. The adjustability of the fluid jet made possible by the device disclosed herein guarantees for each type of electrode an unambiguous and geometrically controllable conveyance position, in particular a threading position of the machining electrode with respect to the work piece to be machined. Under some circumstances, by varying the cross section of the fluid jet, the conveyance suction exerted by the fluid jet on the machining electrode can be varied to permit a controlled influence on the conveyance precision, in particular the precision in threading the wire electrode. Specifically, it is possible with the disclosed device to adapt the threading condition for different wire erosion types and different cutting types so that the tip of the wire electrode assumes a defined position with respect to a starting bore or a working gap in a work piece, so that responsible threading of the wire electrode is guaranteed. This has been possible in the past only by replacing the conventional wire threading nozzle by another suitable nozzle. In the manner disclosed herein wire electrode threading operations are readily possible even in an extremely inclined position of the wire guidance head.
To guarantee the most extensive possible automation in machining, the electrode guidance device is preferably combined with a controller (preferably a CNC controller), of the machine tool for automatically varying the cross section of the feed-through channel as a function of the type of machining electrode and/or the conditions pertaining to the work piece (e.g., the diameter and type of machining electrode, the diameter of a starting bore or the working gap in the work piece, the height of the work piece and/or other conditions involving the work piece).
A method of automatically threading a machining electrode in the form of a wire or strip into a wire erosion machine is also disclosed, where the cross section of the wire threading fluid jet is automatically adjusted as a function of the type of machining electrode and/or the conditions prevailing on the work piece (e.g., the diameter and type of the machining electrode, the diameter of a starting bore or a working gap in the work piece, the height of the work piece and other conditions pertaining to the work piece).
The disclosed wire threading method is preferably combined with a method of automatically changing the machining electrode by using an automatic electrode changer. For example, in the case of a wire erosion machine, an automatic wire changing device is provided, such as that described in German Patent Application 19,963,416.5, the disclosure of which is hereby incorporated by reference in its entirety. At least two devices are provided here for processing wire electrodes from the devices holding a selected wire electrode which is unwound from a storage roll and bringing the wire electrode into a suitable position (in the operating state) for feeding it into the electrode running system of the machine and then releasing the respective selected machining electrode.
In another example, a combination of the disclosed wire threading method with a conventional fluid jet wire threading method is also conceivable. Such an approach is based on the use of a standard nozzle which serves as the wire threading element to produce a fixed threading jet. In this way it is possible to operate in at least two threading modes, namely a standard threading mode with a fixed fluid jet produced with the standard nozzle, and a fine jet mode in which a suitable fine threading jet for extremely fine wire electrodes can be produced with the adjustable electrode guidance device, for example.